The commonly used term for illegally organised raves, usually held in some skanky derelict building/warehouse (but can be outside too) and located by calling a given mobile number after a specified time on the night of the party. Normally has a start time but no end time as it generally goes on until the police get wind and shut it down! Usually a VERY messy affair involving a blend of acid techno/psytrance copious amounts of hedge monkeys and a side order of random chavs.
"I went to an amaaazing free party in that office block last night! Everyone was lovin it til the rozzers shut it down!!"